A method has been developed for the estimation of local rates of protein synthesis in brain in vivo by quantitative autoradiography. The method is based on the use of L-(1-14C) leucine as a tracer for the incorporation or leucine into protein. A four-compartment model for the behavior of leucine in brain has been analyzed, and an equation has been derived that defines the rate of amino acid incorporation into protein in terms of the time course of plasma- specific activity, final tissue concentration of 14C, and lambda, the ratio of the distribution volumes for the labeled and unlabeled leucine. A value of lambda for the brain as a whole has been experimentally determined. A number of studies have been carried out in order to ascertain the potential usefulness of the method. These studies include: 1) normal development; 2) slow wave sleep; 3) barbiturate anesthesia; 4) ketamine anesthesia; 5) electrical stimulation of the sciatic nerve; 6) hypothyroidism; 7) circadian rhythms; and 8) effects of testosterone on regenerating neurones.